gentoo-portage/Dockerfile.template: Use distfiles.gentoo.org for GENTOO_MIRRORS

This is more generic, and it's easy to customize the mirror by using a
package-cache container as a transparent proxy.  The clients can still
think they're using the generic mirror, but you can volume-mount an
/etc/conf.d/package-cache in the proxy container and set SOURCES to
whatever you like.